1. Field of the Invention
The present invention is directed generally to non-aqueous electrochemical cells or batteries of both primary and secondary (rechargeable) types and, more particularly, to a Lithium-based electrochemical cell utilizing a solid polymer electrolyte which has inherent properties that result in operational safety, in addition to high energy density and excellent packaging flexibility. Specifically, the invention involves the formation of a lithium/polymer-based electrochemical cell utilizing a solid gelled film polymeric electrolyte (SPE) system including polyacrylonitrile (PAN) in combination with one or more solvent plasticizers together with one or more lithium salts and, if desired, photoinitiators or crosslinking agents. The electrolyte film is combined with an anode, a composite cathode and current collector in an extremely thin laminar structure and offers configurations of different voltages based on the cathode active material.
2. Related Art
It is known that non-aqueous active metal, particularly lithium, cells have allowed those skilled in the art to achieve much greater energy densities or energy to weight ratios than had been possible with other electrochemical couples. The wide range of potential uses for these cells has led to a great deal of interest in improving the performance and safety of the cells and to develop more reliable primary and secondary or rechargeable cells utilizing these materials. Such cells normally consist of a light, strongly reducing anode, normally an alkali metal such a lithium or a lithium intercalation compound, an aprotic, non-aqueous liquid solvent/depolarizer into which an appropriate quantity of an electrolyte salt of the anode metal has been dissolved to form a conductive electrolyte solution and an oxidizing agent as the cathode material.
Prior cells using lithium intercalation or insertion compounds as anodes are shown, for example, in U.S. Pat. No. 5,147,739 to Beard, and assigned to the assignee of the present invention, and intercalation materials used for the positive cathode electrodes including LiCoO.sub.2, TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, V.sub.6 O.sub.13 and other such compounds are illustrated in U.S. Pat. No. 4,804,596 to Ebner and Lin (a co-inventor in the present invention) also common of assignee. A further patent, U.S. Pat. No. 4,853,304, also to Ebner and Lin and commonly assigned, discloses an improved non-aqueous electrolyte solution for lithium cells which utilizes an organic-based electrolyte system. That system is one in which an organic ester of formic acid, preferably methyl formate, is combined with an amount of lithium salt and an amount of CO.sub.2 to provide improved electrolyte performance, particularly in secondary or rechargeable lithium cells. These cells have been quite successful; however, they all have liquid electrolytes and so uses are limited to applications suitable for cells having liquid electrolyte systems.
More recently, solid polymer electrolytes have been developed for both primary and secondary Li-based cells which utilize thin film polymer-based gelled electrolyte technology to produce cells having a total cell thickness which is typically 8-10 mil (0.02-0.025 cm) and which offer extreme ruggedness and excellent packaging flexibility. Li-based gelled polymer electrolyte cells typically combine a lithium salt and an amount of plasticizer material in a highly viscous polymer matrix which may or may not be further crosslinked.
The utilization of the gelled polymer electrolyte has brought about the development of new concepts in battery technology. The approach is particularly attractive to space applications because of the special features of high energy density and light weight. Until the development of this technology, non-aqueous lithium cells were limited to liquid electrolyte systems that were sensitive to intercell leakage and other problems. In addition to greater flexibility, the solid or semi-solid nature of the Li-based polymer electrolyte cells offers greater safety than conventional liquid electrolyte cells.
The polymer based lithium rechargeable battery technology is faced with key challenges, however, to improve the general characteristics of the polymer electrolyte itself. Conductivity enhancement, especially at low temperature, capacity fading due to interfacial problems between adjacent intracell layers, and the need to reduce or eliminate the degree of anodic passivation due to storage or due to intermittent usage still lie between the known present state of the art and widespread use of such cells for many applications.